There has been a continual need for sintering nonoxide ceramics efficiently, particularly those that are suitable for high-hardness and high-temperature applications, such as are required for drill and tool bits and for ceramic armor. Boron carbide is an excellent lightweight material for ceramic armor and high-wear applications. However, the material does not sinter well, requiring temperatures of 2000.degree. C. or higher for densification with or without hot-pressing. Microwave sintering has shown to be applicable for sintering the material, but the thermal insulating material cannot be yttria grit unless significant contamination by an yttrium-containing phase is acceptable.